1. Field of the Invention
This invention relates generally to electric motor actuated devices of the type having an armature shaft which is rotatably supported by bearings mounted in a housing of the device. The invention relates more particularly to an improved bearing support arrangement.
2. Description of the Prior Art
An armature shaft of an electric motor used with devices such as portable, hand held, electrically powered tools is supported at opposite ends of the shaft by sleeve bearings which are mounted in a housing of the device. One example of such a device is a portable electric, hand held drill. The housing for the device has frequently been fabricated of a thermoplastic material which provides various beneficial characteristics. However, a thermoplastic material is a relatively poor conductor of heat and the housing does not readily permit conduction from the armature bearing of heat generated by the frictional engagement between the bearing body and rotating armature shaft. The bearing body is thus suceptible to operating at an elevated temperature and can experience a relatively shortened operating life.
A support body which both supports the sleeve bearing body in the housing and conducts heat from the bearing body is known. This type of bearing support body provides a heat sink for the bearing body. A flow of thermal energy from the bearing body to the support body and the combined cooling of the bearing and support bodies, principally through convection by flowing air, thus permits the bearing to operate at desired temperatures. One such form or bearing support and heat sink is described in U.S. Pat. No. 3,824,684 which is assigned to the assignee of this invention. Bearing supports of this type depend in large part for the desired cooling effect on the relatively large thermal mass of the support body. In addition to the desired heat conduction and heat dissipation the relatively large mass support body provides a relatively rigid support for the bearing body.
While providing these beneficial characteristics, the heat sink support body also exhibits some other less desirable factors including cost, interface with the housing and structural rigidity. More particularly, these heat sink support bodies are relatively costly in terms of both the quanity of the material required for the support body itself and the cost of fabrication by casting and finishing. Devices of the type employing the heat sink support body are generally made in relatively large production quantities. Unit cost increments for members of the device can be reflected in terms of relatively large dollar amounts in view of large production quantities and thus each incremental increase or decrease in cost can be reflected in relatively large overall increases or decreases for the production quantities involved. The interface between the bearing support body and the thermoplastic housing is also important. The fabrication of prior heat sink support bodies generally requires a careful processing of the support body during fabrication to avoid an assymetrical body which can result in uneven heating of the body. Such uneven heating causes nonuniform stressing of the body and at times a cracking of the thermoplastoc housing. Additionally, while the support body provides a desired rigid support for the bearing, its rigidity can be disadvantageous in that it permits little yield when manufacturing variations occuring in the cavity impede placement of the bearing body therein. Thus, in order to avoid this potential problem, the cylindricity of the cavity in the support body is accurately formed and the cost of the device is increased.
Another form of heat sink which functions both to support and cool the bearing is provided by a plurality of members rather than a single body. The complexity of this multipiece support and heat sink arrangement is undesirable in that the materials, fabrication, and assembly thereof become relatively costly and thermal transfer contact surfaces between the members introduces undesired resistances to the conduction of the heat.